1. Field of the Invention
This invention relates generally to electrical filters and relates particularly to filter apparatus and a method of forming so-called ceramic filters and duplexers.
2. Description of Related Art Including Information
RF ceramic filters are well known in the art. They are constructed of blocks of ceramic material that are typically coupled to other electronic circuitry through discrete wires, cables, and pins coupled to conductive connection points on external surfaces of the blocks. They are also used to construct duplexers and other electronic components.
Ceramic block filters are used in wireless communication products. Three major process steps in the manufacture of these filters are: (1) the capacitive element pattern generation, (2) I/O pad generation, and (3) tuning the filter to the proper operating frequency. The function of the capacitive element pattern on the filter is to approximate the RF response required by the customer. The function of the I/O pad generation operation is to provide the interface from the filter to the wireless communication product. The function of the tuning operation is to finally adjust this approximated RF response to meet the exact customer requirements for the desired response.
In the prior art, a ceramic block is sintered and then silver metallic paste is placed on all sides of the block EXCEPT for those sides that require a defined electrical circuit such as a capacitive element pattern or input/output (I/O) pads. On those sides, the silver metallic paste is applied to the ceramic block in the form and shape of the desired pattern and the I/O pads (through a well-known screen print process technology or abrasion technology). A heating process causes the metallic paste to solidify with the pattern(s) and/or I/O pads generally in their proper location.
However, this screen print process does not have the dimensional accuracy desired in the plating of the capacitive element filter pattern and other filter elements. The capacitive element filter pattern dimensional accuracy on the ceramic block filters is required to be four times (4xc3x97) as accurate at 1.8 GHz as at 900 MHz.
Thus, in order to complete the product, the capacitive element filter pattern must be further tuned to meet exact customer specifications. This can be accomplished by adding excess metallic paste adjacent circuit pattern features and then using some method to sinter the metallic paste, forming an integral addition to the pattern or pads or, in some cases, material can be removed for tuning. By applying a signal to the input and monitoring the output signal during this process, the operator can terminate the addition of material to the pads or terminals when the output signal indicates that proper tuning has been achieved.
Thus, in U.S. Pat. No. 5,198,788 fine tuning of ceramic filter metallic terminals or pads is disclosed. In this patent, the ceramic block is coated on all sides but one (and perhaps a portion of an adjacent side) and on the uncoated side (and portion), silver metallic paste is formed in the general shape of the desired electronic terminals, pattern, or pads. The silver metallic paste is heat-treated to form a rigid coating. Additional silver metallic paste is then placed adjacent and electrically contacting the formed terminals or pads and, while an input signal is being monitored on the output terminal, a laser beam is used to scan the silver metallic paste to sinter it and form a solid addition to the terminal or pad until the proper electrical characteristics of the device is obtained. Thus, this patent relates to the addition of metal to the already generally formed terminals, pattern, or pads to tune the circuit.
In U.S. Pat. No. 5,769,988 there is disclosed a method of manufacturing a ceramic electronic component having a dielectric ceramic and a conductor thereon containing silver as the main component. By heat-treating the device, such as a dielectric resonator formed with that process, the xe2x80x9cQxe2x80x9d value of the resonator is increased. In the process of heat-treating the device, it is disclosed that the device is subjected to a heat treatment at 400xc2x0 C. or more in an atmosphere containing 10% or less by volume of oxygen. Further, Kagata, et al (""988) disclose xe2x80x9cforming the conductive paste . . . in a pattern of electrodesxe2x80x9d on the sintered dielectric ceramic substrate.
U.S. Pat. No. 5,162,760 also relates to electrical filters formed of ceramic blocks using abrasive or miling methods to remove metallization or use various screen-printing techniques to apply conductive materials onto the various surfaces of the ceramic blocks. In the ""760 patent, a layer of conductive material is deposited on the surface of the block and after the layer is successfully cured, portions of the conductive layer are removed by any suitable milling machine such that the desired conductive pattern is left on the surface. Both the conductive material coating the block and the dielectric material are removed from the block in the areas that are milled. This device is limited in accuracy or precision by the electrode dimension that can be formed with a milling machine.
In U.S. Pat. No. 5,379,011 there is described a ceramic band-pass filter with improved input/output isolation and having conductive material removed from the metallization of the block and the I/O pads are deposited in those areas where the conductive metal had been removed. Again, in this patent, all six sides of the ceramic block are metallized with the exception of the top or upper surface and a portion of the side surface. Slots are formed, between the deposited input/output pads and adjacent metal in the ceramic material and thus, when not plated, varies the dielectric between the input/output pads.
It would be desirable to have such a filter with good isolation between conductive areas and having greater dimensional accuracy of the filter pattern and I/O pads than the present art can provide.
U.S. Pat. No. 5,602,518, assigned to the assignee of the present invention, discloses a ceramic block filter having five sides completely metallized and one side completely unmetallized (side 110 in FIG. 1). However, the ""518 patent differs from the present invention in that the channels (108 in FIG. 1) are completely metallized and serve the purpose of increasing the magnetic coupling between resonators. On the contrary, the trenches of the present invention are not metallized and serve the purpose of allowing adjacent metallized areas to be capacitively coupled.
In the present process, the ceramic block is formed in the usual manner. It has at least one planar surface. Then, instead of coating only those sides where the pattern or I/O pads are not to be formed, the ENTIRE ceramic block is coated with a conductive metallic material. One example of such metallic coating is a paste well known in the prior art and containing an electrically conductive metal (such as silver, for example only) and is then subjected to the necessary heat treatment to solidify the metal. Other examples of conductive coatings include plating the ceramic blocks with a conductive metal and the like.
An ablative method, such as the use of a scanning laser beam, is used to remove unwanted metallic material from at least one planar surface to form the desired capacitive element filter patterns. This differs from the abrasive method of the prior art or the screen printing of the prior art. The laser beam ablates both the metallization and a portion of the ceramic block to form trenches that surround metallic filter components and create the pattern in the desired shape. The depth and width of the trenches determine coupling capacitance of the filter and thus determine its operating frequency. The precision and repeatability of forming the trenches with the lasing process allows greater accuracy and repeatability of the capacitive element filter pattern and the other filter components. The more precise patterns allow for higher tune rates, higher factory yields, and more design margin for the product designers.
However, a well-known problem occurs as a result of the ablation process. During the lasing process, the ceramic material is adversely affected and the xe2x80x9cQxe2x80x9d of the ceramic material is reduced to a point where the filter has no commercial value. Therefore a post-lasing, high-temperature heating process is required to restore the ceramic xe2x80x9cQxe2x80x9d back to its approximate original value.
Since, during the ablation process, the capacitive element patterns and other filter components are being formed, a signal cannot be connected to the input pad for monitoring at the output pad to see if the capacitive element patterns and the other filter components being formed are of the correct dimensions. After the filters have been formed, such signals cannot be applied and measured to the product specification because the ceramic block has such a reduced xe2x80x9cQxe2x80x9d that they are only a generalized representation of the signals that would be found in a finished product. Therefore, for a given product specification, a trial-and-error ablation process is used by continuing to make metallized blocks with different dimension conductive patterns until signals representing the proper RF response range have been established to form a xe2x80x9creferencexe2x80x9d ceramic block. Since the lasing process is extremely precise and repeatable, great numbers of the reference device can then be produced and then a high temperature heating process is used to provide the proper RF response once the appropriate patterns have been generated.
Thus, it is an object of the present invention to provide a method of forming electrically conductive metallization patterns on a ceramic block that are electrically isolated from each other by a pattern of dielectric material.
It is still further an object of the present invention to provide a ceramic block that has its entire surface coated with a conductive material and to use a scanning laser beam to ablatively etch unwanted metallic material and corresponding ceramic material from the ceramic block and create trenches that form at least a portion of the pattern of dielectric material that establishes the desired metallization pattern.
It is also an object of the present invention to ablatively etch unwanted metallic material in such a way that a portion of the ceramic block is also removed sufficient to form trenches that electrically isolate adjacent metallic areas formed by the ablative etching.
It is another object of the present invention to ablatively etch unwanted metallic material from a designated surface area of the ceramic block to form trenches of dielectric material that create a desired metallization pattern, including input and output terminals.
It is still another object of the present invention to apply test signals to the input terminal of a post-lased, pre-heated ceramic filter and to monitor the output signal to determine by trial and error when a filter having the desired electrical characteristics is obtained.
It is yet another object of the present invention to heat the ablatively-etched ceramic block in an ambient atmosphere to restore the xe2x80x9cQxe2x80x9d of the ceramic material.
Thus, the present invention relates to a method of forming ceramic block metallization patterns comprising the steps of encasing the entire surface of a ceramic block, including at least one planar surface, with a conductive metal, such as a metallic paste, for example only, solidifying the conductive metallic paste into a metallic material, and ablatively etching unwanted metallic material from a designated surface area of the ceramic block to form a desired metallization pattern, including input and output terminals if desired.
The invention also relates to a method of forming RF ceramic block filters comprising the steps of encasing the entire external surface area of a ceramic block with a conductive metallic material, using a pattern of dielectric material to electrically isolate and form electrically conductive circuit elements on the encased ceramic block, ablatively etching unwanted metallic material and a portion of the ceramic block from a designated surface area of the encased ceramic block to form at least one trench that forms at least a portion of the pattern of dielectric material and heating the ablatively-etched ceramic block to increase the xe2x80x9cQxe2x80x9d thereof.
The invention also relates to trench filters and duplexers in which each conductive element formed in a ceramic block whose entire surface is encased in a conductive material, each of the conductive elements being at least partially surrounded by a trench or recessed area extending through the conductive material and into the ceramic block, the recessed area having a predetermined depth and width to affect the coupling capacitance of the filter or duplexer and thus control an operating characteristic of said filter or duplexer.